The Curtis Mongo was designed to help advancing young sailors (USA Juniors Team and others) stay upwind in sub planing conditions.
It achieved this goal (upwind in planing conditions) to some degree.
It was never intended for use in fully planing conditions when the board works best with a 65-70 cm carbon formula racing fin (Curtis/Deboichet/Drake/Kashy/ etc.).
So, you may be asking the Mongo to do something it was never intended to do.
The principle reason for the Mongo design was for the kids who will race regardless of if the wind is below the planing minimums.
Without the Mongo, they tended to get so far downwind in < 7-8 knots of wind that they could not finish races within the time limit.
Yes, if you are planing (even planing some of the time) the 70 cm Drake is going to be much faster, take you upwind far higher, and will not "wash out" (I suspect this is regular old "spin out") like the Mongo.
At 200 lbs., if you get your technique right, you should be able to handle the 70 cm Drake up to around 25 knots of wind.
When you get a new sail, you might want to get something a little smaller and try out some big free Race sails (Sailworks Retro, Severne Overdrive, NP V8, etc. as these sails probably have better early planing characteristics than 12-12.5 m2 formula race sails.
Also, look for some 66-68 cm formula race fins for when the wind gets stronger and the 70 cm Drake is a bit too much.
Even though you are a big guy, you need to "rig down" when the wind gets beyond the comfortable range for your early planing gear.
Hope this helps,